Power control in a wireless communication system

ABSTRACT

A method of power control in a wireless communication system, wherein blocks are transmitted from a transmitter to a receiver via a wireless transport channel. The method comprises comparing a target signal quality value with a received signal quality value and providing the results of the comparing step to the transmitter to adjust transmit power based on the comparing step. The target signal quality value is set by the following steps: determining an initial target value; detecting if a data block has been received; detecting if received blocks have been successfully decoded; and decreasing the target value when pass blocks are received and increasing the target value when failed blocks are received subject to monitoring a period of inactivity on the transport channel in which no blocks are received.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of, and therefore claims thebenefit of International Application No. PCT/EP2008/067332 filed on Dec.11, 2008, entitled “POWER CONTROL IN A WIRELESS COMMUNICATION SYSTEM,”which was published in English under International Publication Number WO2009/077422 on Jun. 25, 2009, and has priority based on GB 0724421.3filed on Dec. 14, 2007. Each of the above applications is commonlyassigned with this National Stage application and is incorporated hereinby reference in their entirety.

TECHNICAL FIELD

The present invention relates to power control in a wirelesscommunication system.

BACKGROUND

FIG. 1 is a schematic block diagram indicating the main functionalcomponents of a wideband code division multiple access (WCDMA) receiver.Reference numeral 2 denotes an antenna which receives a wirelesstransmission and supplies it in analog form to RF and IF stages 4. Areceiver front end 6 includes the functions of analog to digitalconversion and supplies digital samples to a signal detection block 8.The signal detection block 8 can be implemented in a number of ways andis responsible for de-scrambling and de-spreading the received codedsignal samples. For each time slot a block is received which comprises aplurality of transport channels (TrCH) multiplexed onto a WCDMAdedicated physical channel (DPCH). As shown in FIG. 1, after signaldetection and channel decoding the decoded data bits are supplied to aCyclic Redundancy Check (CRC) block 12. The CRC check indicates whetheror not the data block has been correctly decoded.

For interference-limited wireless systems, such as those based on CDMAtechnology, link adaptation is performed by a Transmit Power Control(TPC) mechanism, which ensures that sufficient but not excessive poweris transmitted to achieve an adequate received signal quality. In a 3GPPWCDMA system, the power control mechanism comprises two parts: 1) aso-called “outer-loop” algorithm 14 that sets and adjusts a targetsignal-to-interference power ratio (SIR) in order to meet a Block ErrorRate (BLER) target set by a network; and 2) a so-called “inner-loop”algorithm 16 that provides fast feedback to the transmitter in orderthat the transmitter can adjust its transmitted signal power so that thereceiver SIR target is met. The inner-loop transmit power control 16 istypically based on the comparison between a target SIR (SIR_(target))and an SIR estimated from the received signal (SIR_(est)). Theouter-loop mechanism 14 increases or decreases the SIR target inresponse to the receipt of block error information, which is typicallyderived by the pass/fail of the CRC check 12. If a data block isreceived correctly (CRC pass) then the SIR target is decreased; if adata block is received incorrectly (CRC fail) then the SIR target isincreased. In a typical implementation, the amount the SIR target isdecreased following a correctly decoded block is equal to some step size(in dB) multiplied by the target block error rate, and the amount theSIR target is increased following an incorrectly decoded block is equalto the step size multiplied by one minus the target block error rate.For example, for a 10% BLER target and a 1 dB step size, the SIR targetwill be decreased by 1*0.1=0.1 dB following a good block and increasedby 1*(1−0.1)=0.9 dB following a bad block. This has the effect that, fortypical BLER targets, many more good blocks are required to lower thetarget than bad blocks to raise it by the same amount. In normalcircumstances, the inner-loop power control is able to adjust thetransmitted power to meet the new target in a short period (in WCDMA thepower can be changed by 1 dB per slot). However, under certainconditions, such as when the transmitter has reached its maximum allowedtransmit power, it may be the case that the SIR achieved at the receiveris lower than the target SIR for an extended period of time such thatmultiple data blocks are received while the condition pertains. In thoseconditions bad (CRC failed) blocks are likely, and the effect of them isto further raise the SIR target progressively higher with respect to theachieved SIR, for no benefit. When conditions return to normal followingsuch a period (e.g., because the receiver has moved closer to thetransmitter), the SIR target will be excessively high and the inner-loopwill adjust the transmit power so that it is higher than necessary toachieve the desired BLER. To avoid such situations a so called“anti-windup” mechanism may be employed to limit how high the SIR targetmay rise above the measured achieved SIR. Following an SIR targetincrease (CRC fail), the anti-windup algorithm inhibits a further SIRtarget increase until the measured SIR has tracked the target variation.Performance requirements for such a mechanism in the User Equipment (UE)receiver are specified in 3GPP TS 25.101, “Technical Specification GroupRadio Access Network: User Equipment (UE) radio transmission andreception (FDD)”, June 2004, Section 8.8.3.

On a 3GPP WCDMA downlink Dedicated Physical Channel (DPCH), TransportFormat Combination Indicator (TFCI) bits (3GPP TS 25.211, “TechnicalSpecification Group Radio Access Network: Physical Channels and Mappingof Transport Channels onto Physical Channels (FDD)”, December 2005,Section 5.3.2) are transmitted to indicate to the UE receiver whichelement of a finite, pre-defined Transport Format Combination Set (TFCS)is employed on the current frame. The Transport Format Combination (TFC)provides information on the number of transport blocks, their size,origin (transport channel), coding scheme, rate matching attributes,etc. for the UE to use in decoding the received data. For high spreadingfactor (i.e. low bandwidth) DPCHs, the number of TFCI bits may be asignificant proportion of the whole transmitted DPCH slot. For instance,for Slot Format 3 (ibid., Table 11) there are 2 TFCI bits per slot outof only 20 bits, corresponding to a 10% overhead. Under certaincircumstances (3GPP TS 25.211, “Technical Specification Group RadioAccess Network: Multiplexing and channel coding (FDD)”, June 2006,Sections 4.3.1 and 4.3.1a) it is permissible to omit the TFCI bits tosave bandwidth, in which case the UE is required to infer which TFC wasused by performing some processing of the received data. This process isknown as Blind Transport Format Detection (BTFD). A special case of BTFDis single transport format detection, in which there are only twopossible transport formats per Transport Channel (TrCH): either no dataor a single block of data (with CRC attached). Although the 3GPPstandard specifies a BTFD performance test (3GPP TS 25.101, “TechnicalSpecification Group Radio Access Network: User Equipment (UE) radiotransmission and reception (FDD)”, June 2004, Section 8.10) whichrequires that false detections (i.e., events where the wrong TFC isidentified) should occur at a rate lower than 10⁻⁴, the test does notapply to single format detection so there is no requirement that thefalse alarm rate be low in that case. Indeed, under certain channelconditions it may be a necessary compromise to have a non-negligiblefalse alarm rate in order to achieve an acceptable detection rate.

Single format BTFD false alarms pose a significant problem to thecorrect functioning of outer-loop power control. It is common inreal-world scenarios to find large periods of time where no data istransmitted on a particular TrCH (e.g., on a TrCH used only forsignaling data). In such conditions, a succession of false alarms, whichappear as blocks of data with bad CRCs, cause the SIR target to increasemonotonically, until blocked by the anti-windup mechanism, as there areno good blocks received on that TrCH to lower the target. Even when theanti-windup mechanism stops the target SIR from rising, the transmittedpower may be higher than necessary. This result has a negative effect oncell performance and may lead to the offending UE having its calldropped by a Radio Resource Management (RRM) algorithm at the basestation transmitter or Radio Network Controller (RNC).

SUMMARY

The aim of the present invention is to provide a power control mechanismwhich obviates or mitigates the foregoing disadvantages.

According to one aspect of the present invention there is provided amethod of power control in a wireless communication system whereinblocks are transmitted from a transmitter to a receiver via a wirelesstransport channel, the method comprising comparing a target signalquality value with a received signal quality value and providing theresults of the comparing step to the transmitter to adjust transmitpower based on the comparing step. The target signal quality value isset by the following steps:

-   -   determining an initial target value;    -   detecting if a data block has been received;    -   detecting if received blocks have been successfully decoded, and        identifying the received blocks as pass or fail blocks; and    -   decreasing the target value when pass blocks are received and        increasing the target value when failed blocks are received        subject to monitoring a period of inactivity on the transport        channel in which no blocks, or only failed blocks are received        and where the period of inactivity exceeds a threshold reducing        a current target value to a value determined by the target value        at commencement of the period of inactivity (SIR_(t) ₁ ). The        value determined by the target value can be a value lower than        target value at commencement of the period of inactivity        (SIR_(t) ₁ −Δ). Alternatively, the target value can be reset to        the same value as the initial target value (SIR_(init)).

According to another aspect of the invention there is provided areceiver for a wireless communications system. The receiver comprises:means for attempting to detect blocks transmitted from a transmitter tothe receiver via a wireless transport channel and detecting if blockshave been successfully decoded, and identifying the blocks pass or fail;means for generating a target signal quality value by decreasing thetarget value when pass blocks are received and increasing the value whenfailed blocks are received; means for comparing the target signalquality value with a received signal quality value and providing theresult of the comparing step to the transmitter to adjust transmit powerbased on the comparing step; and means for monitoring a period ofinactivity on the transport channel in which no blocks, or only failedblocks are received and means operable to adjust the target value sothat where the period of inactivity exceeds a threshold, the currenttarget value is reduced to a value determined by the target value atcommencement of the period of inactivity (SIR_(t) ₁ ).

Another aspect also provides a wireless communication system comprisinga receiver as defined hereinabove and a transmitter wherein thetransmitter is operable to adjust the transmit power based on the resultof comparing the target signal quality value of the received signalquality value.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show how thesame may be carried into effect, reference will now be made by way ofexample to the accompanying drawings, in which:

FIG. 1 is a schematic block diagram of a WCDMA receiver;

FIG. 2 is a schematic block diagram of an embodiment of the presentinvention;

FIG. 3 is a flow chart illustrating a method in accordance with anembodiment of the invention; and

FIGS. 4 to 7 are timing diagrams illustrating the effect on the SIRtarget of implementing embodiments of the invention.

DETAILED DESCRIPTION

Those skilled in the art to which this application relates willappreciate that other and further additions, deletions, substitutionsand modifications may be made to the described embodiments. FIG. 2 is aschematic block diagram of an embodiment of the present invention shownas functional blocks. It will readily be appreciated that in practicethese blocks can be implemented by software or firmware in a suitablyprogrammed processor.

FIG. 2 shows the channel decoding block 10, the CRC check block 12, theouter-loop power control block 14 and the inner-loop power control block16 as in FIG. 1 already discussed. The described embodiment of thepresent invention provides a modified anti-windup mechanism which isparticularly useful in conjunction with single format blind transportformat detection (BTFD). It will readily be appreciated however that theaspects of the invention discussed herein can be used in othercircumstances where there is potential for “false alarms”, i.e.,detection error events where no data is received but the receivermistakenly assumes that a data block has been transmitted, tries todecode it and deems it to be a bad block.

The outer-loop power control block 14 maintains and uses a separate SIRtarget for each transport channel (TrCH) multiplexed onto a dedicatedphysical channel (DPCH). These targets are held in memory block 18.

The set of TrCH SIR targets is initialised to some set of typical values{SIR_(init)} at call setup, where the value of SIR_(init) for aparticular TrCH may depend upon a number of factors related to, forinstance, the properties of the DPCH (e.g., spreading factor) or theproperties of the TrCH (e.g., BLER target). The SIR target used by theinner-loop power control and by the anti-windup mechanism is calculatedfrom these constituent targets (e.g., by taking the instantaneouslargest target).

FIG. 2 also illustrates additional components in accordance with anembodiment of the present invention in the form of a timer function 20,a compare function 22 and a reset function 24.

FIG. 3 is a flow chart illustrating a method of setting the SIR targetfor use by the inner-loop power control 16 using the functionsillustrated in FIG. 2.

For each TrCH, a quantity SIR_(init) is selected to be used as aninitial value of the SIR target for that TrCH. At a step S2, for eachTrCH, a timer is started when the TrCH is initiated. A step S3 performsBTFD, which detects whether a data block has been received. CRCdetection is implemented at a step S4.

For each TrCH, in correspondence to a CRC pass (Y at step S4), theTrCH's current SIR target is recorded (step S5), the SIR target isdecreased (step S6), and the TrCH's timer is reset to zero (step S7).The manner in which the SIR target is decreased is as discussed abovewith reference to FIG. 1. If the TrCH's SIR target is above a certainvalue (for example, SIR_(init)) (step S8), the TrCH's timer is restarted(back to step S2), else the timer remains frozen (step S9) until the SIRtarget reaches a certain value (for example, SIR_(init)) and is thenrestarted.

The maximum allowed value of a TrCH's timer shall be τ_(inactivity), thevalue of which is a design parameter: τ_(inactivity) shall be shortenough that the probability of the SIR target rising excessively issmall (or short enough that the probability of a very negative effectdue to the rise of the SIR target is small) but long enough thatsufficient time is allowed for the SIR target to rise to an acceptablelevel following the reception of actual data which is in error due topoor SIR. When no data is detected at the BTFD detection step (N at stepS3), or in correspondence to a CRC fail (N at step S4), the lapsed timeis compared with τ_(inactivity) (respectively steps S11 and S12). For aCRC fail, when τ<τ_(inactivity) (N at step S12), the SIR target isincreased in a manner as described with reference to FIG. 1 in theintroductory portion hereto (step S13). For both the cases of no dataand CRC fail, when τ_(inactivity) is reached for a TrCH (Y at steps S11and S12), the TrCH's SIR target shall revert to the value recorded atthe start of the inactivity period, or to the value recorded at thestart of the inactivity period less some amount Δ (step S1).

FIG. 4 is a timing diagram illustrating the effect of these steps on thelevel of the SIR target for a particular transport channel. Time t₀represents the start of block transmission, where the SIR target startsinitially at SIR_(init). As can be seen from FIG. 4, the SIR targetrises in steps due to the receipt of bad blocks and then reduces whengood blocks start to arrive. Time t₁ denotes receipt of a good blockprior to a following succession of bad blocks and no data blocks. Eachtime a bad block is received, the SIR target is increased as shown bythe subsequent steps in FIG. 4. However, at time t₂ the lapsed time τhas exceeded the value τ_(inactivity) and the SIR target is reduced tothe value SIR_(t) ₁ which was recorded at time t₁.

FIG. 5 illustrates the timing diagram for an alternative embodiment ofthe invention in which the SIR target is reverted to its value SIR_(t) ₁at time t₁, corresponding to the start of the period of inactivity, lessa certain quantity Δ. A transport channel's SIR target shall not bebelow a certain value (for example SIR_(init)) by the subtraction of Δ.This is shown in FIG. 6, where the difference between the SIR targetSIR_(t) ₁ and SIR_(init) is less than Δ. The value of Δ used is a designparameter—the higher its value the quicker an inactive transport channelwill have its SIR target reduced to SIR_(init).

FIG. 7 is a timing diagram which shows the situation when the value ofthe SIR target SIR_(t) ₁ to which the system reverts at t₂, the end ofthe period of inactivity, is below SIR_(init). In this case, as SIR_(t)₁ <SIR_(init), we do not apply Δ. This represents the option discussedat step S9 in FIG. 3 and the timer is frozen so that on receipt of thenext block there is no further reduction in the SIR target. Thus, onreceipt of a good block, the SIR target is not reduced because it isalready below SIR_(init) (which is shown by the black cross in FIG. 5)and τ is not restarted until the SIR target becomes greater thanSIR_(init).

The timer function 20 can be implemented as a clock measuring a timeperiod or as a counter used to count the number of frames ortransmission time intervals (TTIs) which have elapsed for which no goodblock was detected. This embodiment is described below.

A record is kept for each TrCH of the number of consecutive TransmissionTime Intervals (TTIs) for which no good block (CRC pass) was detected(this includes those TTIs for which no data was detected and those forwhich a CRC fail was detected, either due to poor reception or falsealarm) while a TrCH's SIR target exceeds a certain value (for example,SIR_(init)). The number of TTIs with no CRC passes is a measure of theinactivity of the associated TrCH. A record is also kept of the TrCH'sSIR target at the start of the no-CRC-pass period. If a TrCH's count ofconsecutive TTIs without a CRC pass exceeds some threshold (which wouldbe equivalent to τ_(inactivity)), then the TrCH's SIR target is revertedto its value at the start of the period. In a variation of thisembodiment, if the number of consecutive TTIs without a CRC pass exceedsthe threshold, then the SIR target is reverted to its value at the startof the period minus a predefined value, so that over time the SIRtargets of TrCHs with no detected activity revert to a certain value(for example, SIR_(init)). A TrCH's SIR target shall not be loweredbelow a certain value (for example, SIR_(init)) by this process. If agood block is received for a TrCH, then the count of TTIs without CRCpasses is zeroed and normal outer loop power control is resumed.

One important design parameter of the above embodiment is represented bythe threshold for the number of consecutive TTIs without CRC pass—whichdetermines the period at which the SIR target is reset to its initialvalue (possibly minus a predetermined value). In a WCDMA system, thisperiod can be suitably chosen as a compromise between on one handminimising the amount of time the SIR is raised due to false alarms, andon the other allowing sufficient time for the SIR target to respond to anumber of failed Radio Link Controller (RLC) retransmissions. In aspecific embodiment of the invention, the no-CRC-pass period is set toan integer multiple of a radio frame. For instance, the algorithm canwait for an estimated inactivity of 50 frames, and then reset the SIRtarget to the value at the beginning of the 50-frame period, or to thevalue at the beginning of the 50-frame period minus 0.5 dB.

The advantage of the embodiments of the invention described above isthat the SIR targets of inactive TrCHs are prevented from risingexcessively high due to BTFD false alarms, thus avoiding the necessityof going into anti-windup or having an unnecessarily high SIR target andtransmit power. Further, by allowing the SIR targets of inactive TrCHsto decay over time to some initial value, the effects of those inactiveTrCHs on the combined SIR target are reduced so that transmit power iscontrolled according to the needs of the active TrCHs only. This isparticularly important for the typical case where there are two TrCHs,one carrying a Signaling Radio Bearer (SRB) whose activity is bursty,with large periods of inactivity, and the other carrying a Radio Bearer(RB) with user data. Depending on the block size of the SRB data and thechannel coding scheme used for the SRB transport channel, the SRB mayrequire a higher SIR than the RB. When the SRB is active then the DPCH'sSIR target should be high enough to service the SRB, but when it is notthen the DPCH SIR target should be able to take into account only theRB's target.

What is claimed is:
 1. A method of power control in a wirelesscommunication system wherein blocks of a transport channel aretransmitted from a transmitter to a receiver via a wireless transmissionchannel, there being a period of time where no data blocks aretransmitted for that transport channel, the method comprising comparinga target signal quality value with a received signal quality value andproviding the results of the comparing step to the transmitter to adjusttransmit power based on the comparing step, wherein the target signalquality value is set by the following steps: determining an initialtarget value; detecting if a data block has been received, saiddetecting generating detection error events in said period of time whichappear as fail blocks; detecting if received blocks have beensuccessfully decoded, and identifying the received blocks as pass orfail blocks; and decreasing the target value when pass blocks arereceived and increasing the target value when fail blocks are receivedsubject to monitoring a period of inactivity on the transport channel inwhich no blocks and fail blocks are identified and where the period ofinactivity exceeds a threshold reducing a current target value to avalue determined by the target value at commencement of the period ofinactivity.
 2. A method according to claim 1, wherein blocks aretransmitted via multiple transport channels and wherein an initialtarget value is set for each transport channel and a signal qualitytarget is maintained for each transport channel, the target signalquality value used in the comparing step being derived from the multiplesignal quality target values.
 3. A method according to claim 2,comprising the step of estimating the received signal quality value foreach transport channel.
 4. A method according to claim 2, whereinmultiple transport channels are transmitted on the same physical channelcomprising the steps of estimating one received signal quality value formultiple transport channels.
 5. A method according to claim 1, whereinthe target signal quality value is a signal to disturbance ratio for thechannel, where the disturbance can be interference, noise, orinterference plus noise.
 6. A method according to claim 1, wherein theperiod of inactivity is monitored by a timer.
 7. A method according toclaim 6, comprising the step of resetting the timer when a good block isreceived.
 8. A method according to claim 7, comprising the step of, whena good block is received, comparing the current target value with acomparison target value and resetting and restarting the timer if thecurrent target value is greater than the comparison target value, andresetting and freezing the timer if the current target value is lessthan the comparison target value, in which case the timer will berestarted when the target value becomes greater than the comparisontarget value.
 9. A method according to claim 8, wherein the comparisonvalue is the initial target value.
 10. A method according to claim 1,wherein the period of inactivity is monitored by counting the number oftransmission time intervals during which no good block is detected. 11.A method according to claim 1, wherein the period of inactivity ismonitored by counting the number of radio frames during which no goodblock is detected.
 12. A method according to claim 1, wherein the valuewhich is determined by the target value at commencement of the period ofinactivity is the target value at the commencement of the period ofinactivity.
 13. A method according to claim 1, wherein the value whichis determined by the target value at the commencement of the period ofinactivity is obtained by subtracting an amount from the target value atthe commencement of the period of inactivity.
 14. A method according toclaim 13, comprising the step of checking if the step of subtracting theamount from the target value at commencement of the period of inactivityresults in a value which is less than a comparison value, and if soincreasing the value to the comparison value.
 15. A method according toclaim 14, wherein the comparison value is the initial target value. 16.A method according to claim 1 comprising the step of determining if ablock of data has been received, prior to the detecting step, usingblind transport format detection.
 17. A receiver for a wirelesscommunications system, the receiver comprising: means for detectingblocks of a transport channel transmitted from a transmitter to thereceiver via a wireless transmission channel, there being a period oftime where no data blocks are transmitted for that transport channel,said detecting generating detection error events in said period of timewhich appear as fail blocks; means for detecting if blocks have beensuccessfully decoded and identifying blocks as pass or fail; means forgenerating a target signal quality value by decreasing the target valuewhen pass blocks are received and increasing the value when fail blocksare received; means for comparing the target signal quality value with areceived signal quality value and providing the result of the comparingstep to the transmitter to adjust transmit power based on the comparingstep; and means for monitoring a period of inactivity on the transportchannel in which no blocks and fail blocks are identified and meansoperable to adjust the target value so that where the period ofinactivity exceeds a threshold, the current target value is reduced to avalue determined by the target value at commencement of the period ofinactivity.
 18. A receiver according to claim 17, wherein said means formonitoring the period of inactivity is a clock.
 19. A receiver accordingto claim 17, wherein the means for monitoring the period of inactivityis operable to count the number of transmission time intervals in whichno good block is detected.
 20. A receiver according to claim 17, whereinthe means for monitoring the period of inactivity is operable to countthe number of radio frames in which no good block is detected.
 21. Areceiver according to claim 17, comprising means for storing initialtarget values for multiple transport channels and maintaining separatetarget signal quality values for multiple transport channels, whereinthe target signal quality value which is compared with the receivedsignal quality value is derived from said multiple targets.
 22. Areceiver according to claim 17, comprising means for storing an amountby which the target value at commencement of the period of inactivity isreduced to obtain the value determined by the target value atcommencement of the period of inactivity.
 23. A wireless communicationsystem comprising a receiver according to claim 17 and a transmitter,wherein the transmitter is operable to adjust transmit power based onthe result of comparing the target signal quality value with thereceived signal quality value.
 24. A communications system according toclaim 23, wherein the result of comparing the target signal qualityvalue with the received signal quality value is supplied to thetransmitter via a wireless transport channel.
 25. A receiver for awireless communications system, the receiver comprising: a componentarranged to detect blocks of a transport channel transmitted from atransmitter to the receiver via a wireless transmission channel, therebeing a period of time where no data blocks are transmitted for thattransport channel, said detecting generating detection error events insaid period of time which appear as fail blocks; a component arranged todetect if blocks have been successfully decoded and identifying blocksas pass or fail; a component arranged to generate a target signalquality value by decreasing the target value when pass blocks arereceived and increasing the value when fail blocks are received; acomponent arranged to compare the target signal quality value with areceived signal quality value and providing the result of the comparingstep to the transmitter to adjust transmit power based on the comparingstep; and a component arranged to monitor a period of inactivity on thetransport channel in which no blocks and fail blocks are identified anda component operable to adjust the target value so that where the periodof inactivity exceeds a threshold, the current target value is reducedto a value determined by the target value at commencement of the periodof inactivity.